Bug 1203745 (CVE-2015-2751, xsa127)

ISSUE DESCRIPTION
=================

XSA-77 put the majority of the domctl operations on a list excepting
them from having security advisories issued for them if any effects
their use might have could hamper security. Subsequently some of them
got declared disaggregation safe, but for a small subset this was not
really correct: Their (mis-)use may result in host lockups.

As a result, the potential security benefits of toolstack
disaggregation are not always fully realised.

IMPACT
======

Domains deliberately given partial management control may be able to
deny service to the entire host.

As a result, in a system designed to enhance security by radically
disaggregating the management, the security may be reduced. But, the
security will be no worse than a non-disaggregated design.

VULNERABLE SYSTEMS
==================

Xen versions 4.3 onwards are vulnerable.
Xen versions 4.2 and earlier do not have the described disaggregation
functionality and hence are not vulnerable.

MITIGATION
==========

The issues discussed in this advisory are themselves bugs in features
used for a security risk mitigation.

There is no further mitigation available, beyond general measures to
try to avoid parts of the system management becoming controlled by
attackers. Those are the kind of measures which we expect any users
of radical disaggregation to have already deployed.

Switching from disaggregated to a non-disaggregated operation does NOT
mitigate these vulnerabilities. Rather, it simply recategorises the
vulnerability to hostile management code, regarding it "as designed";
thus it merely reclassifies these issues as "not a bug".

Users and vendors of disaggregated systems should not change their
configuration. The robustness benefits of disaggregation are
unaffected, and (depending on system design) security benefits are
likely to remain despite the vulnerabilities.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.